High in the Simien Mountains, researchers are getting a close-up look at the exotic, socially adventuresome primates known as geladas

Geladas are isolated, oddball monkeys that science has largely overlooked. They live in large herds in the towering Simien Mountains of northern Ethiopia. A few researchers studied the primates in the 1970s, but famine and political turmoil in the region made further investigations impossible. "Almost no one's heard of geladas," says Jacinta Beehner, a University of Michigan biological anthropologist in the midst of a ten-year gelada study, the most extensive ever conducted. "They kind of got lost in the shuffle."

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Baby geladas (one-year-olds at play) are dependent on their mothers for two years.
(Anup Shah and Fiona Rogers)

Young males venture away from the group for a few hours at a time after age 4 and depart at 6; females never leave the harem.
(Anup Shah and Fiona Rogers)

Geladas (a male and female in the Simien Mountains) signal their status with the livid skin on their chests: dark pink conveys dominance. To avoid hyenas and leopards, the nimble primates sleep on narrow cliff ledges.
(Anup Shah and Fiona Rogers)

A male gelada that controls a harem must defend his supremacy (a leader bares his teeth at rivals).
(Anup Shah and Fiona Rogers)

An unattached male (the pale skin patch at his throat indicates he's not currently mating with females) often gangs up with other bachelors to challenge vulnerable leaders. Fights can last for days.
(Anup Shah and Fiona Rogers)

"When the bachelors are done eating, they become bored and want to cause trouble," says Jacinta Beehner, who studies behavior in gelada herds, among the largest nonhuman primate communities.
(Anup Shah and Fiona Rogers)

Though dominant males have privileges, primarily access to mates (a female grooms a reclining male), sisterhood is powerful in its own way. "The females in these harems are all sisters and cousins and mothers and aunts," says Beehner. "They have a bond. If they don't like a male, they will evict him."
(Anup Shah and Fiona Rogers)

The gelada is the only species remaining from a lineage of grazing primates once more common than baboons, says Robin Dunbar, who studied geladas in the 1970s.
(Anup Shah and Fiona Rogers)

A gelada bachelor group acts aggressively on a cliff ledge.
(Anup Shah and Fiona Rogers)

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Yet—if you don't mind heights—geladas (Theropithecus gelada) make intriguing research subjects. With their falsetto cries, explosive barks and soft grunts, geladas have one of the most varied vocal repertoires of all the primates. The noisy herds are relatively easy to follow. Unlike most monkeys, geladas graze primarily on grass. They are usually observable—except at night, when they disappear over the edges of cliffs to sleep on tiny ledges, safe from leopards and hyenas.

Geladas are visually striking, with burning eyes and leathery complexions. Males have vampiric canines, which they frequently bare at each other, and their golden manes are the stuff of shampoo commercials. "They cry out to be photographed," says Fiona Rogers. She and her partner, Anup Shah, visited Beehner's camp in Simien Mountain National Park for a month to photograph the animals.

Male geladas are the size of large dogs, weighing 50 to 60 pounds. Females are about half as big. Both sexes have a bald, hourglass-shaped patch of skin on their chests that telegraphs a male's social status and a female's reproductive stage. Depending on hormone levels, the color ranges from meek eraser pink to fiery red. Males' patches are brightest during their sexual prime, Beehner and her husband, University of Michigan biologist Thore Bergman, have found, and females' chest patches blister when they are in estrus. (They are also called "bleeding-heart baboons," though they are actually monkeys.)

Geladas favor soft, protein-rich fescue grasses, shoveling away with both hands for hours each day. Their fiber-heavy food generates plentiful stool samples, another reason Beehner loves working with them; analysis of their feces can reveal hormone levels. But their eating habits don't doom them to dull lives. "A lot of people talk about vegetarian primates as being boring, because they just sit around and digest all day," Beeh­ner says. Not geladas. They are always signaling and communicating with each other. "It really is one big soap opera," she says.

Beehner's focus is the evolution of social behavior, and geladas are very social. Herds can be enormous—up to 1,200 individuals. But most interactions occur within a harem, composed of a leader male, two to a dozen females and their young. The females are related to each other, and they sometimes turn on the leader if he is grooming them insufficiently, not protecting them or otherwise shirking his duties.

Groups of sullen-looking bachelor monkeys lurk outside the herds. These juveniles are similar to adolescent street gangs, and Chadden Hunter, an Australian researcher who began studying geladas in the late 1990s, dubbed two such groups the "Sharks" and the "Jets," à la West Side Story. Fiona Rogers took such a liking to the bachelors' hangdog looks that her partner says he felt a stab of jealousy. "I was a little worried," Shah says.

Every so often, a leader male, herding his harem and emitting high squeals, challenges a bachelor, which responds with a "roar bark." According to protocol, the leader tears around, raising a miniature dust storm, with the bachelor in pursuit. The ritual concludes when the leader leaps into a tree, rattles the branches and shrieks "Yeow-Yeow-Yeow!" The bachelor skulks away. "They are showing how strong and fit they are," Beehner says. "It's all about showmanship."

But woe to the leader whose posturing is not persuasive. Eventually a bachelor gang targets a harem to take over, and then, Beehner says, the fighting turns ugly. Young gangsters take turns chasing and tiring out the leader until a bachelor contender steps forward. A gladiatorial battle ensues—with hair pulling, scratching and biting—sometimes leaving one animal mortally wounded.

Beehner remembers one fight that lasted three days. (The leader male prudently took breaks to pay court to his females.) It appeared to be a stalemate until a treacherous female edged away from the harem. As her champion looked on, she sidled up to the bachelor. The leader male "didn't even try to prevent it," Beehner recalls. "He just quit."

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